Induction motors are easily controllable and can be powered from commercial power supplies. Accordingly, the induction motors have widely been used to drive industrial machines in a vast range of industrial fields for a long period of time. A constant voltage/frequency (V/F) ratio control has conventionally been employed as a method of controlling the induction motors. This control method can provide an output voltage proportional to a frequency. Subsequently, a sensorless vector control has become widely used on demand for improvements in motor characteristics in low speed ranges, application of torque control and the like.
On the other hand, permanent magnet synchronous motors are free of no-load current and secondary copper loss and accordingly have a higher efficiency as compared with induction motors. Motors having saliency, such as interior permanent magnet synchronous motors, are controllable at a further higher efficiency by using reluctance torque. The sensorless vector control system has also been in widespread use with respect to the permanent magnet synchronous motors.
The sensorless vector control system is generally based on a vector control system with use of a speed sensor or a magnetic pole position sensor and uses an estimate value of speed or magnetic pole position instead of a signal generated by the sensor. In this case, a large number of types of sensorless vector control systems are provided with a speed controller to control a rotating speed of the motor. The speed controller is generally composed into a combination of a proportioning controller and an integrating controller. Optimization of control parameters of the proportional-integral control necessitates an inertia moment value of a load apparatus.
When control parameters of the speed controller are not optimum, for example, when a set value of the inertia moment is smaller than an actual inertia moment of the load apparatus, an undershoot or overshoot appears in the rotational speed of the motor in the case where the rotational speed is accelerated or decelerated in a relatively shorter time. An excessive degree of occurrence of undershoot or overshoot in the rotating speed causes overvoltage or a phenomenon that the motor is once reversed in rotation when it is decelerated to be stopped, or other phenomena. These abnormal states result in cases where the motor control device cannot meet the specifications thereof.
The control parameters of the speed controller need to be optimized in order that the above-described phenomena may be avoided. However, an entire rotating part cannot sometimes be confirmed from outside when inertia moment of the load apparatus is measured. Furthermore, the specification of the load apparatus or the like cannot be obtained even in many cases when it is tried to be obtained for calculation of inertia moment.
In view of the above-described circumstances, it is suggested that inertia moment of the load apparatus be estimated by the motor control device thereby to be used to optimize the control parameters of the speed controller. However, in order that the load apparatus may be driven, torque sufficient to oppose load torque caused with rotation of the load apparatus needs to be generated on a rotating shaft of the motor. Accordingly, the moment inertia can rarely be tuned in a completely unloaded condition in the motor control device but, rather, normally needs to be accurately estimated while some load torque is applied to the rotor shaft.